Intrinsic optical properties

The optical spectra of conjugated polymers are dominated by an intense absorption due to the electric dipole allowed transition between the highest occupied and the lowest unoccupied electronic states. For solid samples the energy of this transition can lie anywhere between about 0.8 and 4 eV. Data for solid samples are listed in Table 9.1 and for solutions and solid samples in Table 9.2 on page 351. 

Polymer PSQEa pttp PITNC PTVf/ CN-PPV POFBT7 PPV PPSg 

The perfection of the polymer chains in polydiacetylene crystals results in an absorption spectrum with a narrow electronic transition and sidebands due to vibrations of the backbone double and triple bonds. The absorption is 

The dependence of the reflection coefficient on n and k allows these quantities to be calculated from the reflection spectrum by use of the Kramers-Kronig relations. It follows from Equation (9.17) that an intense absorption gives a high reflectivity, e.g. the reflection spectrum of PDA-TS, 

Electro-absorption (EA) spectroscopy, where optical absorption is observed under the application of an electric field to the sample, is another method that can distinguish between localised and inter-band excitations. The electric field produces a Stark shift of allowed optical absorptions and renders forbidden transitions allowed by mixing the wavefunctions of the excited states. Excitons show a quadratic Stark (Kerr) effect with a spectral profile that is the first derivative of the absorption spectrum for localised (Frenkel) excitons and the second derivative for charge transfer excitons, i.e. 

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Optical sensors rely on optical detection of a chemical species. Two basic operation principles are known for optically sensing chemical species intrinsic optical property of the analyte is utilized for its detection indicator lor label) based sensing is used when the analyte has no intrinsic optical property. For example, pH is measured optically by immobilizing a pH indicator on a solid support and observing changes in the absorption or fluorescence of the indicator as the pH of the sample varies with time1 20. 

There are two sets of quantities that are often used to describe optical properties the real and imaginary parts of the complex refractive index N = n + ik and the real and imaginary parts of the complex dielectric function (or relative permittivity) e = c + ie". These two sets of quantities are not independent either may be thought of as describing the intrinsic optical properties of matter. The relations between the two are, from (2.47) and (2.48),... 

Photometric accuracy is determined by comparing the difference between the measured absorbance of the reference standard materials and the established standard value. Many solid and liquid standards are commonly used to verify the photometric accuracy of a spectrophotometer. An optically neutral material with little wavelength dependency for its transmittance/absorbance is desirable because it eliminates the spectral bandwidth dependency of measurements. The advantages and disadvantages of various commonly used photometric accuracy standards are summarized in Table 10.6. Even for a relatively stable reference standard, the intrinsic optical properties may change over time. Recertification at regular intervals is required to ensure that the certified values of the standards are meaningful and accurate for the intended use. 

Conjugated conducting polymers consist of a backbone of resonance-stabilized aromatic molecules. Most frequently, the charged and typically planar oxidized form possesses a delocalized -electron band structure and is doped with counteranions (p-doping). The band gap (defined as the onset of the tt-tt transition) between the valence band and the conduction band is considered responsible for the intrinsic optical properties. Investigations of the mechanism have revealed that the charge transport is based on the formation of radical cations delocalized over several monomer units, called polarons [27]. 

The absorption of impurity centres is observed in the transparency domains of semiconductors and insulators, which are limited by their intrinsic electronic and vibrational absorptions. Further, a brief account of the relevant physical processes and an overview of the intrinsic optical properties of these materials and of their dependence on temperature, pressure and magnetic field is given in this chapter. Some semiconductors have been or are now synthesized in quasi-monoisotopic (qmi) forms because of improvements in their physical properties like thermal conductivity. A comparison of their intrinsic optical properties with those of the crystals of natural isotopic composition is also given. The absorption related to free carriers, due mostly to doping is also discussed at the end of this chapter. A detailed account of the optical properties of semiconductors can be found in the books by Yu and Cardona [107] and by Balkanski and Wallis [4]. 

There are basically two ways to achieve high visual transmittance simultaneously with high infrared reflectance. One is the use of the interference effect in all-dielectric multilayers, the other is the use of intrinsic optical properties of electrically conducting films such as Au, Ag, and others which have high infrared reflection with relatively low visual absorption. Their suitability as transparent heat mirror can be improved by antireflection coating for the visible. Figure 25 shows an example for such types of heat mirrors, according to Fan et al. [102]. 

The majority of methods for the continuous optical monitoring of gases can be divided into two groups. In the flrst, the intrinsic optical property of the gas is exploited to sense it. lypical examples include chlorine, methane, carbon monoxide, and nitrogen oxides. In the second method, an indicator is used to transduce the gas concentration into a measurable optical parameter. This approach has frequently been applied when the gas has no useful intrinsic optical property or when it is dissolved in water. Typical examples include oxygen, carbon dioxide, and sulfur dioxide. 

An optical fiber is used as a plain transducer to guide light to a remote sample and return light from the sample to the detection system. Changes in the intrinsic optical properties of the medium itself are sensed by an external spectrophotometer. 

After polymerization, an add (e.g., HF or H3PO4), base (e.g., NaOH or KOH), or organic solvent can be used to dissolve the nanoporous template and isolate the synthesized NTs or NWs. The intrinsic optical properties of the electrochemically synthesized light-emitting polymer NTs or NWs can be controlled by the synthetic conditions such as molar ratio of monomer to dopant, applied current or voltage, synthetic temperature, and type of solvent used for the dissolution of the nanoporous template. 

All conjugated polymers are potentially electrochromic with the intrinsic optical properties determined by the polymer s tt-tt transition, which is characterized by the polymer s bandgap and [5,16,17]. The value of the optical bandgap is determined by the onset of the tt-tt transition for the pol)mer in the undoped state. 

Taking into accoimt the large specific surface area, unique intrinsic optical properties and easy noncovalent interactions with aromatic drug molecules, GO is a potential material for biomedical applications. Further, GO is photoluminescent in the visible and near-infrared (NIR)... 

Conjugated polymers have an electronic band structure. The energy gap (Eg) between the highest occupied n electron band (valence band) and the lowest unoccupied one (conduction band) determines the intrinsic optical properties of the polymers. 

Optical sensors fall into two categories (1) direct sensors, where the analyte is detected directly via some intrinsic optical property (2) indirect sensors, where the... 

Fiber-optic chemical sensors can be divided into two categories based on their structure (a) Intrinsic sensors are based on the analyte s intrinsic optical properties, and (b) extrinsic sensors are based on sensing materials (chemical or biological) immobilized to the fiber surface, with... 

TABLE 1. INTRINSIC OPTICAL PROPERTIES OF SELECTED POLYMERS... 

Single nanostructures pump probe microscopy experiments revealed energy relaxation pathways that are obscured by ensemble averaging. For example, understanding of the intrinsic optical properties of single wall carbon nanotubes (SWCNTs) has been previously hindered primarily by the broad distribution of semiconducting and metallic nanotube types in as-synthesized... 

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